This invention relates to switching-type power converters and in particular to the reduction of power loss and heat dissipation of such converters over a wide load range of operation.
Power converters can be divided into linear type and switching type. The switching converter has the great advantages of light weight, small size and greater efficiency compared with the linear converter.
A typical dc-dc switching converter circuit such as the one shown in FIG. 1 receives input power from input terminals 24, 26 and produces a switching waveform across the primary winding 28 of transformer 32. The electric power entering primary winding 28 is coupled to secondary winding 30 which feeds the power to a rectifying circuit which in turn outputs converted power to load 38.
The basic forward converter topology shown in FIG. 1 is probably the most widely used topology for powers under 300 W. Input capacitor 20 compensates for the inductance of the cable supplying the input dc power at nodes 24, 26.
When switch 40 is turned on by pulse generator 34, the dot end of secondary winding 30 of transformer 32 goes positive with respect to its no-dot end. Diode 42 is forward-biased, and current flows out to lowpass filter 44, 48. The lowpass filter averages the output waveform and diode 46 works like a free-wheeling diode. When switch 40 turns off, the magnetizing energy stored in transformer 32 is reset by reset circuit 36.
A large portion of the power loss and heat dissipation is due to the switching loss of switch 40 (e.g. FET). This switching loss results from the overlapping of non-zero voltage and current waveforms during the switching transient.
In order to reduce the switching loss, the switching transient time and switching frequency should be reduced. But these techniques have some drawbacks. Using a low switching frequency will lead to larger magnetic components such as a larger transformer and a larger inductor. And when the switching transient becomes very fast, the current slope (di/dt) and voltage slope (dv/dt) will become very large and may cause serious noise problems.
One possible way of reducing the switching loss is the zero-voltage switching method. In practice, the zero-voltage switching method is not completely satisfactory in commercial power conversion products. In order to perform the zero-voltage switching function, a special IC or many more components are usually required, thereby increasing the size and production cost of the power converter. In practice, in order to design a resonant path, some bulky passive components are used (such as a large choke) that increase the size and conduction loss of the converter.
Because of the complexity of the resonant circuit and the control loop, tedious calculations and computer simulations are usually needed. This complicates product design and production. For example, a phase-modulated full-bridge topology can be designed to achieve zero-voltage switching only for a narrow band of output loading. When operated outside of its narrow loading range, it enters the hard-switching mode that greatly increases the heat loss. In general, designing a reliable zero-voltage switching circuit that can cover a wide loading range is very difficult. Thus, there is a continuing need to improve the zero-voltage switching method.